One-trip milling system

ABSTRACT

The side tracking system includes a window mill having a full diameter cutting surface and a reduced diameter tapered cutting surface and a whipstock having a ramp engaging the reduced diameter cutting surface. The materials of the whipstock have a first cutablity and the materials of the casing have a second cutability. The reduced diameter cutting surface contacts the whipstock ramp at a first contact area and the full diameter cutting surface contacts the wall of the casing at a second contact area. As weight is applied to the mill, there is a first contact stress at the first contact area and a second contact stress at the second contact area. A cutability ratio is the first cutability divided by the second cutability and a contact stress ratio is the first contact stress divided by the second contact stress. The mill cuts the casing rather than the whipstock by maintaining the product of the cutability ratio and the contact stress ratio less than one. Preferably the height of the reduced diameter cutting surface is greater than the height of the full diameter cutting surface. The ramp includes a plurality of surfaces having different angles whereby the rate of deflection of the mill by the whipstock varies as the mill is lowered into the borehole. In particular, the ramp of the whipstock includes two surfaces having steep angles, one steep angled surface causing the mill to punch through the wall of the casing and the second steep angle surface moving the center of the mill across the wall of the casing.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/021,630 filed Feb. 10, 1998, hereby incorporatedherein by reference, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/642,829 filed May 3, 1996, now U.S. Pat. No.5,771,972, hereby incorporated herein by reference, and is related toU.S. patent application entitled Two Trip Window Cutting System, Ser.No. 572,592, filed Dec. 14, 1995, hereby incorporated herein byreference, and U.S. patent application Ser. No. 08/916,932 filed Aug.21, 1997, now U.S. Pat. No. 5,894,889, hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a method and apparatus for drilling asecondary borehole from an existing borehole in geologic formations andmore particularly, to a tapered window mill and whipstock combinationthat in one trip, can drill a deviated borehole from an existing earthborehole or complete a side tracking window in a cased borehole.

[0004] 2. Background

[0005] Traditionally, whipstocks have been used to drill a deviatedborehole from an existing earth borehole. The whipstock has a rampsurface which is set in a predetermined position to guide the drill biton the drill string in a deviated manner to drill into the side of theearth borehole. In operation, the whipstock is set on the bottom of theexisting earth borehole, the set position of the whipstock is surveyed,the whipstock is properly oriented for directing the drill string in theproper direction, and the drilling string is lowered into the well intoengagement with the whipstock causing the whipstock to orient the drillstring to drill a deviated borehole into the wall of the existing earthborehole.

[0006] Previously drilled and cased wellbores, for one reason oranother, may become non-productive. When a wellbore becomes unusable, anew borehole may be drilled in the vicinity of the existing casedborehole or alternatively, a new borehole may be sidetracked from ornear the bottom of a serviceable portion of the cased borehole.Sidetracking from a cased borehole is also useful for developingmultiple production zones.

[0007] Sidetracking is often preferred because drilling, casing andcementing the borehole is avoided. This drilling procedure is generallyaccomplished by either milling out an entire section of casing followedby drilling through the side of the now exposed borehole, or by millingthrough the side of the casing with a mill that is guided by a wedge or“whipstock” component.

[0008] Drilling a side tracked hole through casing made of steel isdifficult and often results in unsuccessful penetration of the casingand destruction of the whipstock. In addition, if the window isimproperly cut, a severely deviated dog leg may result rendering thesidetracking operation unusable.

[0009] Several patents relate to methods and apparatus to sidetrackthrough a cased borehole. U.S. Pat. No. 4,266,621 describes diamondmilling cutter for elongating a laterally directed opening window in awell casing that is set in a borehole in an earthen formation. The millhas one or more eccentric lobes that engage the angled surface of awhipstock and cause the mill to revolve on a gyrating or non-fixed axisand effect oscillation of the cutter center laterally of the edge thusenhancing the pipe cutting action.

[0010] The foregoing system normally requires at least three trips intothe well in the sidetracking operation. A first stage begins a window inthe casing, a second stage extends the window through use of a diamondmilling cutter and a third stage with multiple mills elongates andextends the window. While the window mill is aggressive in opening awindow in the casing, the number of trips, such as three, to accomplishthe task is expensive and time consuming.

[0011] Typically window mills are designed with a square bottom, i.e. asquare cross-section. As is shown in FIG. 14, a prior art squarebottomed, cross-sectioned mill provides a point of contact between themill and the whipstock and a large axial surface contact between themill and the casing. As can be appreciated from FIG. 14, the contactarea between the square bottomed mill and whipstock is substantially aline contact while the contact area between the mill and casing is muchgreater. The applied force, due to the weight on bit, per contact areadetermines the contact stress between the members. Because the contactstress between the mill and the casing is much greater than the contactstress between the mill and whipstock, the mill tends to cut into thewhipstock rather than into the casing even where the cutability of thewhipstock has been reduced because of hardfacing.

[0012] U.S. Pat. Nos. 2,216,963; 3,908,759; and 4,397,355 disclose millshaving a taper or tapered nose. A starter mill with a tapered nose willeventually wedge and cannot complete the window or drill the lateralborehole. U.S. Pat. No. 3,908,759 appears to disclose a taper on themill. U.S. Pat. No. 2,216,963 discloses a tapered mill which is used ina second trip into the well to increase the window after a squarebottomed mill opened the window in a previous trip into the borehole.These patents do not teach guiding and moving these tapered millslaterally through the casing so that at least the center of thedownwardly facing cutting surface of these mills passes outside theexterior wall of the casing in one trip into the borehole. At least twotrips are required into the well, typically using a starter mil in thefirst trip to begin cutting a window in the casing and then a secondmill in a second trip to increase the window. Further, tapered mills aretypically less than full gauge requiring additional into the borehole tocomplete the window.

[0013] Weatherford Enterra offers a mill which has a taper extendingupwardly and inwardly from a full diameter cutting base. The mill alsoincludes a support shoulder on the cutting face of the mill. However,the reduced diameter taper extends above the full diameter cutting gageof the mill which therefore tends to cut the whipstock rather than thecasing.

[0014] U.S. Pat. No. 5,109,924 teaches a one trip window cuttingoperation to sidetrack a wellbore. A deflection wedge guide ispositioned behind the pilot mill cutter and spaced from the end of awhipstock component. The shaft of the mill cutter is retained againstthe deflection wedge guide such that the milling tool frontal cuttingsurface does not come into contact with the ramped face of thewhipstock. In theory, the deflection wedge guide surface takes over theguidance of the window cutting tool without the angled ramp surface ofthe whipstock being destroyed.

[0015] However, when a second and third milling tool attached to thesame shaft as the window milling cutter and spaced, one from the otheron the support shaft contacts the whipstock ramped surface, they millaway the deflection guide projection from the ramp surface. Thisinhibits or interferes with the leading pilot mill window cutter fromsidetracking at a proper angle with respect to an axis of the casedborehole and may cause the pilot window cutting mill to contact the rampsurface of the whipstock before the pilot window cutter mill clears thecasing. The reamers or mills aligned behind the pilot window mill,having the same or larger diameter than the diameter of the pilot windowmill, prevents or at least inhibits the window pilot mill from easilyexiting from the steel casing. This difficulty is due to the lack ofclearance space and flexibility of the drill pipe assembly making up theone trip window cutting tool when each of the commonly supported reamermills spaced along the shaft, sequentially contact the window in thesteel casing. Hence, the sidetracking apparatus tends to go straightrather than be properly angled through the steel pipe casing.

[0016] U.S. Pat. No. 5,445,222 teaches a combination whipstock andstaged sidetrack mill. A tapered, cone-shaped mill is located on the endof a common shaft and has an outer diameter of about 50 to 75 percent ofthe maximum diameter to which the final sidetracked hole will becompleted. Three stages of cutting mills are disposed above the taperedmill on the common shaft. Each successive stage increases in diameter. Asurface of a second stage cutter is, at its smallest diameter, about thediameter of the maximum diameter of the tapered mill, and is, at itslargest diameter, at least 5 percent greater in diameter than thediameter of the tapered mill. A surface of a final stage cutter mill is,at its largest diameter, about the final diameter dimension, and at thesmallest cutting surface diameter, is a diameter of at least about 5percent smaller than the final diameter dimension. The whipstock guideis made of a material that is harder than the casing but not as hard asthe cutting elements of the mill whereby the mill is to cut the casingrather than the whipstock.

[0017] The sidetracking mill is designed to accomplish the millingoperation in one trip. The mill however, tends to go straight andpenetrate the ramped surface of the whipstock. Substantial damage to thewhipstock occurs and sidetracking may not occur as a result.

[0018] While the intent is to perform a sidetracking operation in onetrip, difficulties often arise when attempting to deviate the drillstring from its original path to an off line sidetracking path.Progressively larger in diameter reaming stages to enlarge the window inthe steel casing inhibits the drill shaft from deviating or flexingsufficiently to direct the drill pipe in a proper direction resulting indamage to the whipstock and misdirected sidetracked boreholes. In otherwords, the sidetracking assembly tends to go straight rather thandeviating through the steel casing.

[0019] The present invention overcomes these deficiencies in the priorart.

SUMMARY OF THE INVENTION

[0020] The side tracking system of the present invention includes awindow mill having a tapered cutting surface which allows the mill toinitiate the cutting of a window into the casing and to move the centerof the downwardly facing cutting surface of the mill laterally throughthe window and past the exterior wall of the casing in one trip into thewell without substantially cutting up the whipstock. The tapered cuttingsurface of the window mill includes taper from a full diameter cuttingsurface to a reduced diameter cutting surface adjacent the downwardlyfacing bottom cutting surface of the mill. The mill preferably is usedin combination with a whipstock having a ramp which engages the taperedcutting surface of the mill forming a large contact area between themill and whipstock. The materials of the casing have a first cutablityand the materials of the whipstock have a second cutability.

[0021] The tapered cutting surface contacts the whipstock ramp at afirst contact area and the full diameter cutting surface of the millcontacts the wall of the casing at a second contact area. As weight isapplied to the mill, there is a first contact stress at the firstcontact area and a second contact stress at the second contact area. Theratio of cutability of the mill with the whipstock and casing is thefirst cutability divided by the second cutability and the ratio of thecontact stress of the mill with the whipstock and casing is the firstcontact stress divided by the second contact stress. The mill of thepresent invention cuts the casing rather than the whipstock bymaintaining the product of the cutability ratio and the contact stressratio less than one. This also causes the height of the tapered cuttingsurface to be at least 50% of the total height, the total height beingthe distance from the top of the largest diameter cutting surface on themill to the bottom of the mill.

[0022] An object of the present invention is to achieve a cutabilityratio times the contact stress ratio of the mill with the whipstock andcasing which is less than one such that the mill tends to cut the casingrather than the whipstock. Thus it is a further objective to maximizethe contact area between the mill and the whipstock such as by having atapered cutting surface on the mill and a ramp on the whipstock whichhas angle substantially the same as the taper of the tapered cuttingsurface on the mill. Additionally, the contact area is maximized bycausing the height of the tapered cutting surface to be at least 50% ofthe total height of the mill which is the height of the tapered cuttingsurface and the full diameter cutting surface.

[0023] It is an object of this invention to provide a side trackingsystem which will deflect and move the tapered mill laterally throughthe casing so that at least the center of the downwardly facing cuttingsurface of the mill passes outside the exterior wall of the casing inone trip into the borehole. Further it is an object to provide a sidetracking system in two trips or less and preferably a one trip cuttingsystem for cutting a deviated hole in an existing earth borehole.

[0024] It is another object of this invention to provide a one tripwindow cutting system for cutting an opening in a pipe casing forsubsequent side tracking drilling operations.

[0025] More specifically, it is an object of this invention to provide amill with a tapered cutting end which matches the ramp angle of thewhipstock face such that in operation, as the drill string is rotateddownwardly, the face of the whipstock forces the tapered cutting end ofthe window mill out through the pipe casing. The angled face of thewhipstock adjacent to the window cutting mill and the cutter mill itselfis hardfaced to minimize damage to both the whipstock and the cutermill.

[0026] A one trip side track window cutting apparatus for cuttingsidetracking windows in a casing positioned in previously drilledboreholes consist of a window cutting mill affixed to an end of a shaft,a body of the mill forming a tapered cutting end.

[0027] A whipstock forms a ramp, the angle of which substantiallyparallels an angle of the tapered cutting end of the window mill. Theramp acts as a bearing surface for laterally forcing the window millinto the pipe casing. The face of the whipstock changes the rate ofdeflection of the window mill into the pipe casing.

[0028] The whipstock upstream end is ramped about 15° to match a 15°taper at the end of the window mill cutter. The whipstock upper end isattached to the end of the window mill cutter at the 15° interfacethrough a shear bolt extending from a blade of the window mill forinstallation of the whipstock in a cased borehole. The end of thewhipstock is heavily hardfaced, especially adjacent the interface withthe window cutter mill. Another mill is positioned upstream of thewindow mill on the same supporting shaft and is preferably the samediameter as the window mill. When the shear bolt is sheared through anupward force on the drilling string after the whipstock is anchored andproperly oriented in the cased borehole, the hardfaced ramp formed bythe end of the whipstock forces the window mill immediately into thewall of the casing. Simultaneously, the second mill spaced from thewindow mill is forced into the casing thus starting two openings in thecasing. The whipstock face below the 15° ramp parallel the walls of thecasing for a distance to allow both the window mill and the second millto cut the window started by the initial 15° ramp. As the window cuttingprocess proceeds, the ramp surface of the whipstock transitions into a“normal” 3° ramp for a sufficient distance for the window mill to extendabout half way out of the casing where the ramped surface of thewhipstock transitions again to a more aggressive angle to further urgethe window mill out of the casing.

[0029] Once the window mill is centered on the wall of the casing,further cutting becomes difficult because of the reduced rotation of thecutting edges at the center of the tapered window mill. At the exactcenter of the tapered window mill, there is essentially zero rotation.Thus, in the prior art, it took a long cutting time to have the windowmill move and cut past its center line. On a standard 3° whip face, itoften took a drilling length of plus or minus ten inches to have thecenter line of the window mill cross the wall of the casing. Very slowdrilling progress is made during this period of time because the windowmill is attempting to cut the wall of the casing with essentially zerorotation at the center of the window mill.

[0030] It is advantageous for all of the mills to be full gage. Oneadvantage is that with your window mill being full gage, the window holewill also be full gage when drilling is stopped with the assembly. Ifthe window mill is undergauged, then when the drilling bit is run intothe well, the full gage drilling bit is going to slow down as it cutsthe under gage borehole to full gage. This then slows down theoperator's ability to kick off and drill the new borehole with thedrilling bit. The drilling bit must remount the bottom section of theborehole cut by the window mill. If the hole is full gage, they will beable to use the whip to help build an angle faster and apply weight tothe drilling bit to drill laterally the new borehole. If they have to godown and remount the hole, then they are much further down in the holebefore they can kick out for their lateral drilling.

[0031] The window mill tapers conform to most of the ramp angles formedby the whipstock. For example, the largest diameter of the window millforms a 3° cutting section matching the 3° section of the whipstockbelow the cylindrical portion of the whipstock. Of course, the 15° angleof the window mill is parallel to the 15° formed at the top of thewhipstock. These matching angulations minimize damage to the whipstockface during the window cutting process thereby assuring a successfullycut window in the casing of the borehole.

[0032] After both the window mill and the second mill cut completelythrough the casing, the window mill is tripped out of the borehole. Thesidetracking drilling operation then commences.

[0033] An advantage then of the present invention over the prior art isthe use of a tapered window mill with a surface contour matching theramp angle formed at the upstream end of the whipstock such that themill is forced into the casing immediately after the window mill isreleased from the whipstock without damage to the whipstock.

[0034] Another advantage of the present invention over the prior art isthe formation of angled and parallel ramp surfaces formed on thewhipstock to facilitate and enhance the cutting action of both thewindow mill and the second mill, upstream of and spaced from the windowmill.

[0035] Still another advantage of the present invention over the priorart is the use of an acutely angled ramp section at a point along theramped whipstock surface when the center of the window mill reaches theinside diameter of the wall of the casing resulting in a slowdown in thewindow cutting operation. The “kick out” ramp more quickly moves thetapered window mill past this phase of the window cutting process thusspeeding up the completion of the sidetrack window.

[0036] Other objects and advantages of the present invention will appearfrom the following description.

DESCRIPTION OF THE DRAWINGS

[0037] For a detailed description of a preferred embodiment of theinvention, reference will now be made to the accompanying drawingswherein:

[0038]FIG. 1 is a partial cross-sectional view of a prior artsidetracking operation depicting setting an anchor for a typicalwhipstock sidetracking system in a cased borehole.

[0039]FIG. 2 is a partial cross-sectional view of a first stage of theprior art sidetracking operation illustrating cutting a window sectionin a pipe casing with a typical starter mill.

[0040]FIGS. 3A and B are a partial cross-section of a preferredembodiment of the invention whereby the top of the whipstock matches thetaper of the window mill.

[0041]FIG. 4 is an enlarged partial cross-section of the tapered windowmill illustrating the hollow shear pin attaching the tapered window millto the parallel ramped surface formed adjacent the top of the whipstock.

[0042]FIG. 4A is an enlargement of the tapered window mill of FIG. 4showing contact areas between the mill, casing, and whipstock.

[0043]FIG. 4B is a free body force diagram showing the forces applied tothe assembly of FIG. 4.

[0044]FIG. 5 is a perspective view of the tapered window mill with chipbreaking cutter elements attached to the cutting face of each blade ofthe window mill.

[0045]FIG. 6 is a partial Cross-section of the one trip sidetrack windowcutting apparatus wherein the mill is sheared from the top of thewhipstock and is moved laterally through the casing by 15° ramp angleformed in the top of the whipstock.

[0046] FIGS. 7 are a partial cross-section of the window mill andupstream “tear drop” cutter cutting the window in the pipe casing. Theramp section immediately below the 15° ramp formed in the whipstock isparallel to the axis of the pipe casing while the tear drop cuttercompletes its initial cut in the window from its entry into the casingto its intersection with the cut made by the tapered window mill.

[0047] FIGS. 8 are is a partial cross-section of the window millcontacting a second “kick out” ramp formed in the 3° ramp portion of thewhipstock, the kick out ramp serves to force the window mill out of thecasing so that it will complete the window more efficiently.

[0048]FIGS. 9A and B are a partial cross-section of an alternativewindow cutting apparatus identical to the apparatus shown with respectto FIGS. 6 through 8 with the exception of a “watermelon” millpositioned upstream of the tear drop mill.

[0049]FIGS. 10A and B are a partial cross-section of the alternativeapparatus illustrating the watermelon mill starting its cut into thepipe casing above the window started by the downstream mills.

[0050]FIGS. 11A and B are a partial cross-section of the alternativeapparatus after the window, tear drop and watermelon mills have cut anelongated window in the casing.

[0051]FIG. 12 is a partial cross-section of an alternative whipstockwith a “kick out” ramp in the 3° ramp portion.

[0052]FIG. 13 is a view taken through 13-13 of FIG. 12.

[0053]FIG. 14 is a diagrammatical representation of a prior art squarebottom mill showing contact areas.

[0054]FIG. 15 is a diagrammatical representation of an alternative sidetracking system of the present invention with a mill having a roundedprofile.

[0055]FIG. 16A is a diagrammatical representation of the mill of thepresent invention with a prior art whipstock having no ramp at its upperend.

[0056]FIG. 16B is a diagrammatical representation of the mill of FIG.16A with the tapered mill having cut a taper in the face of the priorart whipstock.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] Referring now to the prior art of FIG. 1, the casing sidetracksystem generally designated as 10 consists of a drill collar 12 attachedto a starter mill 14. The starter mill 14 is affixed to the end of thewhipstock 16 through a shear bolt block 15. The whipstock 16 has ananchor 18 attached to the down hole end of the whipstock. The entireassembly 10 is tripped into a borehole 9 cased with steel pipe casing11. The casing 11 has an interior annular wall having an inside diameterD_(I) and an exterior annular wall having an outside diameter D_(O).After the sidetracking system reaches a desired depth in the borehole,the whipstock 16 is oriented to a desired sidetrack angulation and setor anchored in the steel pipe casing 11. Casing 11 generally is made ofsteel but may be made of various other materials such as fiberglass forexample.

[0058] With reference to the prior art of FIG. 2, once the system 10 isproperly oriented and set in the casing 11, the starter mill 14 isreleased from the end of the whipstock 16 by breaking the solid shearpin 22 secured to the bolt block 15. The starter mill 14 is subsequentlydirected into casing 11 by shear bolt block 15 along ramped surface 17formed by whipstock 16. The starter mill 14 then mills a window 20through the wall of the casing 11. After the starter mill 14 begins thewindow 20, it is tripped out of the cased borehole 9.

[0059] Turning now to the preferred embodiments represented in FIGS. 3through 8, FIGS. 3A and B illustrate a one trip mill assembly generallydesignated as 30 and a whipstock assembly generally designated as 60that includes a whipstock 44. The mill assembly 30 includes a taperedwindow mill generally designated as 32. The mill 32 is attached to thebottom end of a shank or shaft 31. Upstream and spaced from the windowmill is, for example, a second mill 33 also mounted to the shaft 31. Theupstream end of the shaft 31 is either threadably connected to a drillstring or threaded to another subassembly (see FIGS. 9 through 11). Atubular member 27 may form the shaft 31 on which mills 32 and 33 aremounted. Tubular member 27 may include a lower reduced diameter portionon which mill 32 is disposed with mill 33 being disposed on the filldiameter of tubular member 27. This reduction in diameter providesflexibility between mills 32 and 33 during the milling process.

[0060] A third mill may be mounted to a shaft upstream of second mill33. The third mill is desirable in some circumstances and will bediscussed in detail with respect to FIGS. 9, 10 and 11.

[0061] Referring now to FIGS. 3 through 5, the window mill 32 includes aplurality of blades, such as blade 34, having a particular cuttingprofile. Each blade 34 has, for example, a multiplicity of cuttingelements such as tungsten carbide cutters 42 with “chip breakers” formedon the face of the cutters. The chip breakers on the face of each cutterserves to break up the curled cuttings resulting from the window mill 32cutting through the pipe casing 11 so that the cuttings may betransported up the drill string annulus by the mud circulated throughthe drill string. Without the chip breaker, the continuous cuttingscreate a “rats nest” downhole and cannot be easily removed. These highlyeffective cutters, are manufactured by Rogers Tool Works, Rogers, Ark.and are known as Millmaster. It wold be obvious to utilize natural orpolycrystalline diamond cutters (not shown) on the cutting blades 34 ofthe tapered window mill 32 without departing from the spirit of thisinvention.

[0062] Blade 38 immediately adjacent the parallel surface 45 ofwhipstock 44 is preferably wider to accommodate the shear bolt 39threaded into the blade 38. The head of the shear bolt 63 is seated inthe end of the whipstock 61 and the threaded shank 54 is threaded intoblade 38. The shank 54 of the shear bolt is preferably hollow so that,once the bolt 39 is sheared, the shank 54 serves as a nozzle extensionfor nozzles 69 positioned at the base of shank 54 and at the entrance toconduit 37 that directs fluid to the whipstock anchor (not shown). Itwould be obvious however to utilize a shear bolt with a solid shankwithout departing from the scope of this invention.

[0063] The blades 34 of window mill 32 form a radial or lateral cuttingsurface which includes the profile of three cutting surfaces, namely alower tapered cutting surface 52, a medial cutting surface 43, and afull diameter cutting surface 53. As defined, the radial cutting surfacedoes not include the back tapered surface 55 above full diameter cuttingsurface 53. The tapered cutting surface of mill 32 is defined as thatportion of the radial cutting surface which forms an angle with the axis29 of mill 32 and as shown in the preferred embodiment, includes lowertapered cutting surface 52 and medial tapered cutting surface 43. Itshould be appreciated that although mill 32 is shown as having twotapered cutting surfaces 43 and 52, mill 32 may have a common taper ormay have three or more different tapers.

[0064] The blades 34 also form a downwardly facing bottom cuttingsurface 57. Bottom cutting surface 57 is generally flat and circularhaving a diameter which is at least 30% and preferably 65% of thediameter of the full diameter cutting surface 53. This sized bottomcutting surface 57 provides stability to cutting operation of the mill32.

[0065] The lower tapered cutting surface 52 of the window mill 32 istapered, for example, 15° with respect to the axis 29 of the window mill32 and the casing 11 in the borehole. The taper may be in the range ofan angle A from 1 to 45° with respect to the axis 29. The height oftapered cutting surface 52 measured along the axis 29 is L₃. A shear pin39 anchors the tapered window mill 32 through a connection in blade 38of the mill 32 to profiled end surface 45 of whipstock 44. The endsurface 45 of the whipstock 44 is profiled (angle 15°) to match theangle of the lower tapered end 52 of the window mill (15°) ashereinafter described.

[0066] The medial cutting surface 43 has a reduced taper of 3° whichconforms to the 3° tapers on the profiled ramp surface 28 of thewhipstock 44. The taper of surface 43 may be in the range of 1 to 15°with the axis 29. The height of medial taper 43 measured along the axis29 is L₂.

[0067] The final full diameter cutting surface 53 extends verticallyabove medial cutting surface 43 and is parallel to the axis 29. Theheight of full diameter cutting surface 53 measured along the axis 29 isL₁. Full diameter cutting surface 53 is the full diameter of the mill32, i.e. it is the major (largest) diameter of mill It should beappreciated that the full diameter of mill 32 is preferably at least 75%or greater of the full diameter of casing 11 or of the maximum diameterto which the final sidetracked borehole will be completed and still morepreferably is substantially full gauge. Full gauge is defined as themaximum diameter of a mill which can pass down through the casing 11.

[0068] The full diameter cutting surface begins at the first fulldiameter of the mill 32 as one moves down the profile of the mill 32from top to bottom. This is the first point where the mill 32 reachesits full diameter. In the preferred embodiment, the full diameter isbelow tapered back surface 55. The height of the radial cutting surfaceis the distance from the top of the full diameter cutting surface 53,i.e. the top of the largest diameter surface of mill 32, to the bottomof the tapered cutting surface adjacent downwardly facing bottom cuttingsurface 57. This height equals L₁+L₂+L₃.

[0069] The tapered cutting surface, i.e. lower tapered end 52 and medialcutting surface 43, are under full diameter since their diameter is lessthan that of full diameter cutting surface 53. It is preferred that theheight of the full diameter cutting surface 53 of the mill 32 be atleast 3% and no more than 70% of the radial cutting surface of mill 32.Thus, L₁ is less than 70% of the sum of L₁ +L₂ +L₃. It is even morepreferred that the height of the tapered cutting surface be greater thanthe height of the full diameter cutting surface of mill 32. Stateddifferently, the tapered cutting surface, i.e. L₂ +L₃, be at least 50%of the total radial cutting surface height, i.e. L₁+L₂ +L₃. Preferablythe full diameter cutting surface 53 have a sufficient height so as toallow some wear on the full diameter blades 34 and still maintain fulldiameter cutting. Such sufficient height is approximately 3 to 20% ofthe total radial cutting height.

[0070] Referring now to FIGS. 3A and 3B, the whipstock 44 has a diameterD_(W) which approximates the inside diameter D_(I) of the interior wallof casing 11 which allows whipstock 44 to be lowered through casedborehole 9. Whipstock 44 also includes a profiled ramp surface 28 havinga curved or arcuate cross section and multiple surfaces, each of themultiple surfaces forming its own angle with the axis 26 of whipstock44. Profiled ramp surface 28 includes a starter surface 45 having asteep angle preferably 15°, a vertical surface 46 preferably parallel tothe axis 26, an initial ramp surface 47 having a standard anglepreferably 3°, a “kick out” surface 48 having a steep angle preferably15°, and a subsequent ramp surface 49 having a standard angle preferably3°. It should be appreciated that these angles may vary. For example,the starter ramp surface 45 may have an angle A in the range of 1 to45°, and preferably in the range of 2 to 30°, and still more preferablyin the range of 3 to 15°, and most preferably 15°. The vertical surface46 has a length approximately equal to or greater than the distancebetween mills 32 and 33.

[0071] Surface 45 may be heavily hardfaced with, for example, acomposite tungsten carbide material 51 metallurgically applied to theramp surface. Moreover, the entire profiled ramp surface 28 of thewhipstock 44, exposed to the cutting action of the mills, may behardfaced.

[0072] When the window mill 32 is full gage, the “kick out” ramp surface48 begins at that point on the initial 3° ramp surface 47 where thethickness of the ramp surface 47 is approximately equal to the radius ofthe whipstock 44. In other words, the radial distance between that pointon surface 47 and the inside diameter D_(I) of the wall of the casing 11should be approximately the same or slightly greater than the radius ofthe window mill 32. This ensures that “kick out” ramp surface 48 willincrease the rate of deflection of the window mill 32 just before thecenter 25 of the bottom cutting surface 57 of window mill 32 reaches theinside diameter D_(I) of the wall of the casing 11. The “kick out” rampsurface 48 forms an accelerator ramp which exerts a lateral force to thewindow mill 32 and greatly increases the rate of deflection of thewindow mill 32 into the wall of the casing 11. Although the preferredangle of “kick out” surface 48 is 15°, the angle may be from 10 to 45°.It should be appreciated that the kick out ramp surface 48 may be usedin constant angle whipstocks such as a whipstock having a standard rampsurface of, for example, 2 to 3°, with the “kick out” ramp surfacehaving a substantially greater ramp angle located at approximately themid-whip position of the whipstock thereby creating a jog or deviationin the otherwise constant angle of the whipstock. The use of the “kickout” ramp surface 48 allows the design of the window mill 32 toincorporate a lighter dressing which will increase formation ROP.

[0073] The backside 62 of the whipstock 44, especially adjacent theupper end 61 of the whipstock 44, is contoured to conform to the insidediameter D_(I) of the interior wall of the pipe casing 11 for stabilityof the top of the whipstock 44. The opposite lower end of the whipstock44 is secured to a, for example, hydraulically actuated anchor (notshown). A typical anchor is shown in U.S. Pat. application Ser. No.572,592 filed Dec. 14, 1995, incorporated herein by reference.

[0074] The mill 32 and whipstock 44 of the present invention areconfigured such that the mill 32 tends to cut the wall of the casing 11and not the whipstock 44. To achieve this objective, various factors aretaken into consideration including the contact area and contact stressbetween the mill 32, casing 11 and whipstock 44 and the cutability ofthe metal of the casing and of the metal used for the whipstock 44.Various ones of the physical properties of the materials of the casing11 and whipstock 44 determine their cutability, i.e. their resistance tocutting. Cutability is not a particular property such as hardness but isa combination of properties. Cutability is developed through the testcutting of the materials for the whip 44 and for the casing 11. Thelower the cutability number the harder the material is to cut.

[0075] To insure that the mill 32 cuts the casing 11 rather than thewhipstock 44, the assembly must achieve the following formula:

C*(AF _(W) /CA _(W))=AF _(C) /CA _(C)

[0076] Where CA_(W) is the contact area between the whipstock 44 andmill 32;

[0077] AF_(W) is the applied force on the contact area CA_(W) of thewhipstock 44;

[0078] CA_(C) is the contact area between the casing 11 and mill 32;

[0079] AF_(C) is the applied force on the contact area CA_(C) of thecasing 11; and

[0080] C is the ratio of the cutability of the whipstock 44 to thecutability of the casing 11.

[0081] Since contact stress CS is the applied force AF divided by thecontact area CA, CS=AF/CA, and therefore CS_(W)=AF_(W) /CA _(W) andCS_(C)=AF_(C)/CA_(C). Substituting:

C*(CS _(W) /CS _(C))<1

[0082] Thus, the mill 32 will more easily cut the casing 11 before thewhipstock 44 so long as the cutability ratio times the contact stress ofthe whipstock 44 divided by the contact stress of the casing 11 is lessthan one. One result of the contact stress equation is that it ispreferred that the height of the full diameter of the mill 32 be lessthan the height of the under full diameter of the mill 32. As indicatedpreviously, being full diameter does not mean the mill necessarily isfull gauge.

[0083] Referring now to FIG. 4B, making some simple assumptions, a freebody force diagram is shown for the milling assembly of FIG. 4A. W.O.B.is the weight applied to the mill 32. The operator controls the weighton bit force. The applied force AF_(C) of the casing 11 is shown appliedto the full diameter cutting area 53. The applied force AF_(W) of thewhipstock 44 is shown applied to the lower tapered end 52 and is acomponent of the W.O.B. determined by the angle A. It can be seen thatthe contact stress is geometry dependent.

[0084] The smaller the ratio C of the cutability of the whipstock 44 tothe cutability of the casing 11, the larger the ratio of the contactstresses can be between the mill 32, casing 11 and whipstock 44 and havethe mill 32 cut the casing 11 better than the whipstock 44. Thus, it ispreferred that the material of the whipstock 44 have a low cutability.An ideal situation would be to have the whipstock made of a mate al suchas tungsten carbide while the casing 11 is made of steel to reduce theratio C. Further, a lower cutability ratio allows the height of the fulldiameter cutting surface to be increased such that the height of thefull diameter cutting surface may be greater than the height of theunder gauge cutting surface. A higher cutability ratio will require alower contact stress ratio to insure that the product of the ratios isless than one.

[0085] The tapered contact between the mill 32 and whipstock 44 providesa horizontal side component force which is applied to the casing 11. Theangle of contact A between the whipstock 44 and the mill 32 determinesthis side component which equates to the horizontal component of theapplied force on the contact area. Setting the sum of all forces to zeroand assuming no resistance to bending, AF_(C)=W.O.B.*(1/Tan A) andAF_(W)=W.O.B.*(1/Sin A). The smaller the angle A, the larger the sideload components AF_(C) and AF_(W). The object is to keep the contactarea CA_(C) between the casing 11 and the mill 32 to a minimum. As themilling progresses, CA_(C) increases until the mill 32 reaches theoutside wall of the casing 11. Once the mill 32 breaks through thecasing 11, the contact area CA_(C) begins to reduce.

[0086] Referring again to FIG. 4A, the equation may be applied to thepreferred embodiment. If both the materials of the whipstock 44 and thecasing 11 are assumed to be the same, then the cutability ratio C is 1and no longer is a factor in the equation. If C is 1, then the contactstress CS_(W) of the whipstock 44 must be less than the contact stressCS_(C a)on the casing 11 to prevent the mill 32 from cutting away thewhipstock 44.

[0087] Applying the equation to FIG. 4A, and assuming a W.O.B. of 5000lbs and an angle A of 15°, then AF_(C)=18,660 lbs and AF_(W)=19,319 lbs.If CA_(W)=10 in² and CA_(C)=5 in², then CS_(C)=3732 psi and CS_(W)=1932psi. Inserting these into the equation, thenC*(CS_(W)/CS_(C))=1*(1932/3732)=0.5<1.

[0088] Referring to FIG. 14, there is shown a prior art mill. Againassuming W.O.B. is 5000 lbs but with a square bottom mill and awhipstock with a taper of 3°. Calculating the applied forces,AF_(C)=95,406 lbs and AF_(W)=95,537 lbs. With CA_(C)=10 in² and CA_(W)=1in², then CS _(C)=9,541 psi and CS_(W)=95,537 psi. Inserting these intothe equation, then C*(CS_(W)/CS_(C))=1*(95,537/9,541) =10 >1. With theratio of the contact stresses being greater than 1, the prior art squarebottom mill will cut the whipstock rather than the casing.

[0089] The preferred angle A will vary depending upon various factorsincluding the cutability of the casing 11 land whipstock 44. By makingthe contact area between the mill 32 and the whipstock 44 large, thecontact stress between the mill 32 and whipstock 44 is low. Theobjective is to achieve a contact stress ratio which is as low aspossible. Any ratio less than 1 will accomplish the objective of cuttingthe casing 11 over the whipstock 44.

[0090] The present application is directed to the interaction of themill 32, whipstock 44, and casing 11. One objective is to maximize thecontact area between the mill 32 and the whipstock 44 and to minimizethe contact area between the mill 32 and the casing 11 during criticalstages of the milling operation. It was intended that the contractstresses on the casing 11 be higher so that the casing 11 would be cutby the mill 32 rather than the mill 32 cutting away the whipstock 44.Thus, the objective is to have sufficient contact area between the mill32 and whipstock 44 to ensure that the contact stresses between the mill32 and the casing 11 are greater causing the casing 11 to be cut ratherthan the whipstock 44.

[0091] The mill 32 of the present invention may have various crosssectional cutting profiles so long as the contact areas with the casing11 and whipstock 44 produce the preferred contact stresses. Theobjective is to configure the contact stresses between the mill 32,casing 11, and whip stock 44 so that the casing 11 will be cut away.Referring now to FIG. 15, there is shown a mill 70 having a roundedcutting surface 72. Assuming the cutability ratio to be one, so long asthe contact stress between the mill 70 and whipstock 74 is greater thanthe contact stress between the mill 70 and casing 11, the casing 11 willbe cut more than the whipstock 74.

[0092] In operation, the assembly 30 is lowered into cased borehole 9 toa predetermined depth. The whipstock 44 is then rotated to a desiredsidetrack direction followed by hydraulically actuating the anchor (notshown) by directing drilling fluid or “mud” down the drill string 12under high pressure through flex conduit 37 connected to a coupling 35on the end of the window mill 32. Coupling 35 includes a weakened areatherearound such as a reduced diameter portion allowing coupling 35 tobreak cleanly from the mill 32. The pressurized fluid then entersconduit 50 formed in the whipstock 44 and from there to a connectingmember 19 and then to the anchor to extend the pipe gripping elementswithin the anchor (not shown).

[0093] Referring particularly to the enlarged FIG. 4A, once the anchoris set, weight/tension is applied to the drill string 27 impartingsufficient forces to break the shear pin 39 freeing the tapered windowmill 32. The mill 32 is then rotated and lowered to make contact withthe whipstock 44 and casing 11. The relatively steep profiled angle A(15°), formed in surface 45 of the whipstock 44, immediately provides alateral force to the tapered end 52 of the mill 32 thus forcing therotating mill 32 into the interior of the wall of the pipe casing 11 tostart forming a first window 20A in the pipe casing 11.

[0094] The upstream second mill 33, which may be tear drop in shape, isalso forced into the wall of the pipe casing 11 thereby simultaneouslycutting a second window 20B above the first window 20A formed by thewindow mill 32. The surface 46 formed by the whipstock 44 below angledsurface 45 is preferably parallel to the axis of the pipe casing 11while the window mill 32 and the second mill 33 cut simultaneous windows20A and B (FIG. 6).

[0095] With specific reference to FIG. 7, once the upstream window 20B(cut by the second mill 33) merges with the downstream window 20Astarted by the window mill 32, cutting forces are lessened. The rampsurface 47 formed by the whipstock 44 below the parallel surface 46 thentransitions into a ramp with a 3° angle.

[0096] Referring now to FIG. 8, when the center 25 of the bottom cuttingsurface 57 of the window mill 32 starts cutting at the inside diameterof the wall of the casing 11 as the window milling apparatus progressesdown the whipstock 44 and out through the window 20 cut into the pipecasing 11, the cutting or pipe milling action is slowed considerably. Atthis point the “kick out” ramp 48 (15° as compared to the 3° rampsurface 47) “kicks” the window mill 32 out through the casing 11 formore efficient milling of the casing 11. Once the center 25 of mill 32passes from the interior to the exterior of the casing 11 and this partof the window milling process is overcome, the ramp 49 below the kickout ramp 48 reverts back to the standard 3° ramp angle surface 49.

[0097] An alternative embodiment is illustrated in FIGS. 9 through 12. Asecond subassembly generally designated as 56 is positioned intermediatemill assembly 30 and the drill string 12. A third mill 58, such as awatermelon mill, is spaced between the male and female ends of the shankor shaft 59 (FIG. 9).

[0098]FIG. 10 illustrates the third mill 58 having generally the samediameter as the window mill 32 and second mill 33 and serves to bothlengthen the window 20 penetrating the casing 12 above the window 20 cutby the window and second mills 32, 33. It is preferred that all threemills 32, 33 and 58 be full gage.

[0099] The third mill 58 also serves to dress the window opening 20 asshown in FIG. 11 for easy transition of the following side track drillbit assembly.

[0100] The elongation of the window 20 by the watermelon mill 58 isdesirable to facilitate sidetracking drill bit assemblies that arerelatively stiff and the angle of the side track borehole is slight. Alonger window then would be necessary.

[0101] Where the side track angle is more severe and the drill bit sidetrack assembly is relatively limber, a shorter window will suffice andthe watermelon assembly 56 is omitted from the window cutting apparatusas is shown with respect to FIGS. 3 through 8.

[0102] Upon assembly, mill assembly 30 is connected to whipstockassembly 60 by shear bolt 39 with the lower tapered end 52 of windowmill 32 being engagingly disposed against starter surface 45. Further,hydraulic hose 37 is connected to assemblies 20, 30.

[0103] In operation, the whipstock assembly 20 and mill assembly 30 areconnected to the lower end of a drill string 12 and lowered into casedborehole 9 as shown in FIGS. 9A and B. Once the desired depth is reachedfor the secondary or deflection bore, the whipstock assembly 20 isaligned and oriented within the cased borehole 9 and the anchor is setthereby anchoring the whipstock assembly 20 within the cased borehole 9at the desired location and orientation. Tension is then pulled on drillstring 12 to shear shear bolt 39.

[0104] The mill assembly 30 is then rotated and lowered on the drillstring 12. The complimentary lower tapered end 52 on the rotating windowmill 32 cammingly and wedgingly engages starter surface 45 on whipstock44 thereby causing the window mill 32 to kick out and engage the wall ofthe casing 11 thereby forcing the cutting elements 34 into millingengagement. As the window mill 32 rotates and moves downwardly, thewindow mill 32 continues to be deflected out against the wall of thecasing 11 and eventually punches through the wall of the casing 11. Itis important that the starter surface 45 and its center line match thatof the initial surface 52 on the window mill 32. The angle of taperedend 52 and starter surface 45 may be up to 45°.

[0105] Once initial punch out has been achieved, weight on the drillstring 12 is required to push the window mill 32. It is the “punchthrough” of the window mill 32 that is the most important cutting. Oncethe window mill 32 punches through the wall of the casing 11, a ledge iscreated allowing the whipstock 44 to then guide the mill assembly 30through the window 20 cut in the wall of the casing 11.

[0106] This initial guidance of the starter surface 45, the largecontact area, and the hard facing 51 ensures that the whipstock 44 isnot badly damaged by the window mill 32 and that the window mill 32properly initiates the required window cut. It is important to deflectthe window mill 32 away from the ramp surface 20 of the whipstock 44 toavoid the window mill 32 from milling the whipstock 44.

[0107] Referring now to FIGS. 10A and B, once the initial punch out ismade through the wall of the casing 11 by the window mill 32, the windowmill 32 has past the starter surface 45 and is adjacent the straightsurface 46 which allows the mill 32 to run along a straight track. Oncethe window mill 32 moves past the starter surface 45, window mill 32continues to mill the wall of the casing 11 while the second mill 33expands the window in the wall of the casing 11 previously cut by thewindow mill 32. As the second mill 33 follows behind the window mill 32and begins to cut into the wall of the casing 11, there is formed anuncut portion of the casing 11 between the two mills 32, 33 which hasnot yet been milled. As the window mill 32 is lowered downwardlyadjacent to straight surface 42, the second mill 33 cuts the unmilledportion of casing 11 which extends between mills 32, 33.

[0108] If the second mill 33 is deflected into the casing 11, then thatportion of tubular member 27 between the window mill 32 and pilot mill33 may engage the uncut portion of the casing wall which has not yetbeen milled out. If the window mill 32 maintains the steep angle of thestarter surface 45, it is possible that that portion will engage theuncut portion of the wall of the casing 11 and prevent the mills 32, 33from cutting the wall of the casing 11. It is possible that the millassembly 30 could bind and hinder further milling. This is prevented bystraight surface 46 which has a height substantially equal to or greaterthan the distance between mills 32 and 33.

[0109] Upon the window mill 32 moving past the straight surface 46, anyuncut portion of the casing wall between the mills 32, 33 has now beencut by the second mill 33. At this point, the medial surface 43 ofwindow mill 32 engages the ramp surface 47 and the window mill 32 isagain deflected outwardly against the wall of casing 11 to enlarge thewindow 20 and is guided by the surface 47 into the wall of the casing 11without causing any damage to the whipstock 44. Now that the window mill32 has punched through the wall of the casing 11, it begins cutting intothe cement. The second mill 33 is now passing along the straight surface46 and cutting the window 20 that has already been started by the windowmill 32 to make the window wider. As can be appreciated, watermelon mill58, following the second mill 33, also begins cutting and widening thewindow 20 through casing 11. There may be one or more additionalwatermelon mills above the first watermelon mill 58. The purpose of thewatermelon mills is to elongate the top of the window 20 in the casing11 and clean up the window 20 particularly if there has been a ledgecreated.

[0110] Referring now to FIGS. 11A and B, upon completing the millingalong the surface 47, the casing wall will be underneath the window mill32 and the center 25 of the window mill 32 is approaching the insidediameter of casing 11. At this point, the window mill 32 engages kickout surface 48 to assist the crossing of the wall of the casing 11. Thesteeper angle on surface 48 causes the center 25 of window mill 32 tomore quickly kick out and radially pass from the inside diameter to theoutside diameter of the wall of casing 11. The second mill 33 andwatermelon mill 58 are following and expanding and clearing the windowin the wall of the casing 11. The mill assembly 30 drills faster intothe formation once the window mill 32 completely passes the cased walland into the formation.

[0111] The kick out wedge surface 48 is a second steep surface to assistin moving the window mill 32 from the inside diameter to the outsidediameter of the wall of the casing 11. When the center line 25 of thewindow mill 32 is sitting on the wall of the casing 11, the window mill32 is essentially at zero rotation. The purpose for the kick out surface48 is to reduce the drilling time required to cross the wall of thecasing 11. The increased angle of surface 48 allows the window mill 32to move quickly across the wall of casing 11. By increasing the anglebetween window mill 32 and whipstock 44, the cutting distance of thewindow mill 32 is shortened for the center line 25 of the window mill 32to cross the wall of the casing 11.

[0112] Further, additional weight can be applied to the drill string 12to increase the force on the window mill 32 and to cause the center 25of the bottom cutting surface 57 of the window mill 32 to cross thecasing wall more quickly. Once the center 25 of the window mill 32crosses the wall of the casing 11, the window mill 32 goes back to thefinal three degree surface 49 departure to exit. This reduced drillingtime and distance allows significant savings.

[0113] Upon the window mill 32 moving past the kick out surface 48, thecenter 25 of window mill 32 has passed outside of the wall of the casing11 and is creating a diverted path to form a side track through the wallof the casing 11 and a window borehole in the formation. At this point,the medial surface 43 of window mill 32 engages the lower surface 49 oframp surface 20 and the window mill 32 is deflected laterally to drillthe window borehole. The window mill 32 is now being guided by the lowersurface 49 into the formation. The window mill 32 in effect drills thewindow borehole for the drill bit so that the drill bit can get a fasterstart in drilling the new borehole.

[0114] The window 20 is cut substantially the entire length of thewhipstock 44. Once the milling or cutting of the window is completed,the drill string 12 and mill assembly 30 are replaced by a standarddrilling apparatus for drilling the new borehole.

[0115] Turning now to the alternative embodiments of FIGS. 12 and 13, awhipstock generally designated as 144 has, formed on its 3° ramp surface147, a kick out ramp 148.

[0116] The aggressive angle of the ramp 148 formed in the whipstockguide surface 147 enables the conventional window mill cutter 132 toquickly move beyond that part of the milling process which occurs whenthe center 25 of the mill 132 is passing over the wall of the casing 109as heretofore described.

[0117]FIG. 13 illustrates the window mill 132 passing over the wall ofthe casing 109 as it progresses through window 120. The window mill 132need not have a tapered end as does mill 32 in the embodiment of FIGS.1 - 11. This mill 132 may have a leading end with an angle in the rangeof 0 to 45°.

[0118] The ramp angles for ramps 45, 48 and 148 may be from 1 to 45°with respect to the axis of the whipstocks 44 and 144 without departingfrom the scope of this invention.

[0119] Moreover, where parallel surfaces are mentioned such as bladesurface 52 formed by tapered mill 32 and ramp surfaces 45, 48 and 148formed by whipstock 44, these surfaces are considered “substantially”parallel when such surfaces are less than 3° from being exactlyparallel.

[0120] It should also be noted that the pipe casing 11 lining theborehole 9 may be other than steel.

[0121] Moreover, there may not be any casing lining the borehole 9. Manyof the unique features of this invention set forth above will still beadvantageous in successfully drilling a deviated borehole in an existingearth borehole.

[0122] Referring now to FIGS. 16A and 16B, the tapered mill of thepresent invention may be used with practically any whipstock. Althoughit is preferred that the whipstock have a ramp which has substantiallythe same angle as the taper of the tapered cutting surface of the milland that the ramp be of sufficient duration or length that it deflectsthe mill 32 through the casing 11, the tapered mill will cut its owncontact area in the upper end of the whipstock so as to achieve acontact area as it progresses down the borehole that will cause thecutability ratio times the contact stress ratio to be less than one.

[0123] It should be noted that the contact area of the whipstock can becreated by the mill itself even though there is no tapered surface onthe whipstock. It suffices to say that the mill must be of a geometrysuch that it can in fact create the necessary surfaces on the whipstock.For example, the whipstock must have a sufficient thickness so as toallow the mill to cut the necessary contact area.

[0124]FIG. 16A illustrates a tapered mill 80, substantially identical tomill 32, in contact with the upper terminal end 82 of prior artwhipstock 84. Although the upper terminal end of many prior artwhipstocks has a small chamfer or taper, whipstock 84 is shown with ablunt upper terminal end 82 for purposes of illustration. It can be seenthat there is only line contact between mill 80 and whipstock 84 suchthat the contact area 86 between the mill 80 and casing 11 issubstantially greater than the line contact 88 between the mill 80 andwhipstock 84. Thus, the contact stress ratio of the contact stressbetween the mill 80 and whipstock 84 and between the mill 80 and casing11 will be over one and therefore the mill 80 will cut the whipstock 84rather than the casing 11.

[0125] Since the upper terminal end 82 of the whipstock 84 is squaredoff, when the mill 80 is brought into contact with the top of thewhipstock 84, the mill 80 will mill the whipstock 84 as mill 80progresses downwardly thereby increasing the contact area between themill 80 and the whipstock 84. Initially, the mill 80 only contacts thewhipstock 84 at a very small contact area. Therefore, the mill 80 willcut the whipstock 84 rather than the casing 11. The mill 80 willcontinue to cut the top of the whipstock 84 until the cutting of thewhipstock progresses a sufficient amount to increase its contact areasuch that the mill 80 initiates the cutting of the casing 11. Eventuallythe mill 80 will cut a taper into the whipstock 84 as shown in FIG. 16B.It should be appreciated that the contact stresses, and thus the contactstress ratio, will change as the mill 80 progresses downwardly in theborehole 9. The contact stress ratio will decrease as the mill 80enlarges its contact area with the whipstock 84. The mill 80 alwaysmills the casing 11 to some degree while in engagement with the casing11, but as the contact area of the mill 80 and whipstock 84 increases,the cutting of the casing 11 by the mill 80 is increased and the cuttingof the whipstock 84 is reduced.

[0126] Referring now to FIG. 16B, the mill 80 is shown having cut ataper or ramp 90 in the surface of whipstock 84 such that the contactarea has now increased and the contact stress ratio is less than onewhereby the mill 80 will begin to cut the casing 11 rather than thewhipstock 84. The previous position of the upper terminal end of thewhipstock 84 is shown in dotted lines. As mill 80 progresses downwardlyand is deflecting outwardly by whipstock 84, the window is cut in casing11.

[0127] There are many configurations and profiles which will achieve theobjectives of the present invention, not just those shown in the presentapplication. See, for example, U.S. patent application Ser. No.09/021,630 filed Feb. 10, 1998, hereby incorporated herein by reference;U.S. patent application Ser. No. 08/642,829 filed May 3, 1996, now U.S.Pat. No. 5,771,972, hereby incorporated herein by reference; U.S. patentapplication entitled Two Trip Window Cutting System, Ser. No. 572,592,filed Dec. 14, 1995, hereby incorporated herein by reference; and U.S.patent application Ser. No. 08/916,932 filed Aug. 21, 1997, now U.S.Pat. No. 5,894,889, hereby incorporated herein by reference.

[0128] It will of course be realized that various modifications can bemade in the design and operation of the present invention withoutdeparting from the spirit of the spirit thereof. Thus, while theprincipal preferred construction and mode of operation of the inventionhave been explained in what is now considered to represent its bestembodiments, which have been illustrated and described, it should beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically illustrated anddescribed.

What is claimed:
 1. A side track cutting apparatus for cutting asecondary borehole through the wall in an existing borehole, comprising:a cutting tool affixed to the end of a shaft, the cutting tool having afull diameter cutting surface and a reduced diameter cutting surface;and a whipstock having a ramp; said reduced diameter cutting surfacecontacting said ramp at a first contact area and said full diametercutting surface contacting the interior of the wall of the borehole at asecond contact area; and said first contact area being greater than saidsecond contact area.
 2. The apparatus of claim 1 wherein a weight isapplied to said cutting tool creating a first contact stress betweensaid reduced diameter surface and said ramp at said first contact areaand a second contact stress between said full diameter cutting surfaceand the interior of the wall of the borehole, the ratio of said firstcontact stress divided by said second contact stress being less thanone.
 3. The apparatus of claim 1 wherein said whipstock has a firstcutability and the wall of the borehole has a second cutability andwherein a weight is applied to said cutting tool creating a firstcontact stress between said tapered cutting surface and said ramp atsaid first contact area and a second contact stress between said fulldiameter cutting surface and the interior of the wall of the borehole, afirst ratio of said first cutability divided by said second cutabilityand a second ratio of said first contact stress divided by said secondcontact stress, said first ratio times said second ratio being less thanone.
 4. The apparatus of claim 1 wherein said reduced diameter surfacehas a first height and said full cutting surface has a second height,said first height being greater than said second height.
 5. Theapparatus of claim 1 wherein said reduced diameter surface has a firstheight and said full cutting surface has a second height, said firstheight being at least 30% of said first and second heights.
 6. A onetrip side track window cutting apparatus for cutting sidetrackingwindows in a casing having an inside and outside diameter and a secondcutability, the casing being positioned in previously drilled boreholescomprising; a window cutting mill having a full diameter cutting surfaceand a reduced diameter tapered cutting surface; and a whipstock having aramp with a taper adjacent to an end and a first cutability, said ramphaving a reduced thickness adjacent said end and increasing to anenlarged thickness; said reduced diameter cutting surface contactingsaid ramp at a first contact area and said full diameter cutting surfacecontacting the inside diameter of the casing at a second contact area;and said cutting tool creating a first contact stress between saidreduced diameter cutting surface and said ramp and a second contactstress between said full diameter cutting surface and the insidediameter of the casing, a first ratio of said first cutability dividedby said second cutability and a second ratio of said first contactstress divided by said second contact stress, said first ratio timessaid second ratio being less than one.
 7. The apparatus of claim 6wherein said ramp has an angle in the range of 1 to 45° with respect tothe axis of the whipstock.
 8. The apparatus of claim 6 wherein said ramphas an angle in the range of 2 to 30° with respect to the axis of thewhipstock.
 9. The apparatus of claim 6 wherein said ramp has an angle inthe range of 3 to 15° with respect to the axis of the whipstock.
 10. Theapparatus of claim 6 wherein said reduced diameter cutting surface has afirst height and said full diameter cutting surface has a second height,said first height being greater than said second height.
 11. Theapparatus of claim 6 wherein said reduced diameter cutting surface has afirst height and said full diameter cutting surface has a second height,said first height being at least 30% of said first and second heights.12. The apparatus of claim 6 wherein the tapers of said ramp at said endand said reduced diameter cutting surface are substantially the same.13. The apparatus of claim 6 wherein said mill has a radius and saidenlarged thickness of said ramp plus said mill radius are larger thanthe outside diameter of the casing causing said mill to move laterallythrough the wall of the casing.
 14. The apparatus of claim 6 whereinsaid full diameter cutting surface has a diameter over 75% of the insidediameter of the casing.
 15. The apparatus as set forth in claim 1further comprising a second mill, said window mill and said second millbeing affixed to a shaft, the second mill being spaced upstream of thewindow mill, the second mill substantially simultaneously cutting intothe casing when the window mill is laterally directed into the casing.16. The invention as set forth in claim 15 wherein the diameter of thesecond mill is about the same as the diameter of the window mill. 17.The invention as set forth in claim 16 further comprising a third millaffixed to a shaft, the third mill being spaced from the second mill andserving to elongate the window cut by the window mill and the secondmill, the third mill also serving to dress the window formed in thecasing.
 18. The apparatus as set forth in claim 17 wherein the thirdmill is a watermelon shaped mill with about the same diameter as thewindow mill and the second mill.
 19. The invention as set forth in claim16 wherein the whipstock forms a ramp surface below said end of thewhipstock that is substantially parallel to the axis of the whipstock,the non-angled whipstock ramp surface allowing the window mill and thesecond upstream mill to simultaneously cut a window, when the secondmill window cut merges with the window cut formed by the window mill,the parallel ramp surface transitions into a slightly angled ramp todirect the window mill and the second upstream mill further out throughthe casing.
 20. A method of drilling a window in a casing having aninside and outside diameter and disposed in a well comprising: disposinga first cutting member and a whipstock within the casing; engaging afirst contact surface on the first cutting member with a first rampsurface on the whipstock; engaging a second contact surface on the firstcutting member with the interior of the casing; applying a force to thefirst cutting member creating a first contact stress at said firstcontact area and a second contact stress at said second contact area;providing a first ratio of the cutability of the whipstock to thecutability of the casing and a second ratio of the first contact stressto the second contact stress with the first ratio times the second ratiobeing less than one; and passing the centerline of the first cuttingmember from the inside diameter to the outside diameter of the casing.21. A cutter apparatus for cutting a window through the wall in anexisting borehole, comprising: a body having a generally flat bottomcutting surface, a full diameter cutting surface, and a reduced diametercutting surface disposed between said bottom cutting surface and saidfull diameter cutting surface; said full diameter cutting surface havinga first axial height and being substantially full gauge; said reduceddiameter cutting surface having a second axial height; said second axialheight being at least 30% of said first plus said second axial heights;and said bottom cutting surface having a diameter which is at least 30%of the diameter of said full diameter cutting surface.
 22. A method ofdrilling a window in a casing comprising: engaging a tapered cuttingsurface on a mill with a whipstock in the cased borehole; increasing thecontact area between the mill and whipstock until the contact stressbetween the mill and whipstock is less than the contact stress betweenthe mill and casing; and cutting a window in the casing.
 23. The methodof claim 22 further comprising deflecting the center of a bottom cuttingsurface of the mill from the interior to the exterior of the casing. 24.The method of claim 22 wherein the cutability of the whipstock is lessthan the cutability of the casing.
 25. A side track cutting apparatusfor cutting a secondary borehole in an existing borehole, comprising: acutting tool affixed to the end of a shaft, the cutting tool having atapered cutting surface; and a whipstock having a ramp with an anglesubstantially the same as the angle of the tapered cutting surface fordeflecting the cutting tool into the wall of the existing borehole anddrilling the secondary borehole.
 26. A one trip side track windowcutting apparatus for cutting sidetracking windows in a pipe casingpositioned in previously drilled boreholes comprising; a window cuttingmill affixed to an end of a shaft, a body of the mill forming a taperedcutting end; and a whipstock forming a ramp, the angle of whichsubstantially parallels an angle of the tapered cutting end of thewindow mill, said ramp acting as a bearing surface for laterally forcingthe window mill into the pipe casing.
 27. The apparatus of claim 26wherein the ramp angle is in the range of 1 to 45 degrees with respectto the axis of the whipstock.
 28. The apparatus of claim 26 wherein theramp angle is in the range of 2 to 30 degrees with respect to the axisof the whipstock.
 29. The apparatus of claim 26 wherein the ramp angleis in the range of 3 to 15 degrees with respect to the axis of thewhipstock.
 30. A one trip side track window cutting apparatus forcutting sidetracking windows in a pipe casing positioned in previouslydrilled boreholes comprising; a substantially full gage window cuttingmill affixed to an end of a shaft, a body of the mill forming a taperedcutting end, and a whipstock forming a ramp adjacent to an end, theangle of which substantially parallels an angle of the tapered cuttingend of the window mill, commencement of the window cutting process inthe pipe casing results in the angled whipstock ramp laterally forcingthe window mill into and through the pipe casing, the parallel rampacting as a bearing surface during the initial window cutting operation.31. The invention as set forth in claim 25 further comprising a secondmill affixed to a shaft, the second mill being spaced upstream of thewindow mill, the second mill substantially simultaneously cutting intothe pipe casing when the window mill is laterally directed into the pipecasing.
 32. The invention as set forth in claim 31 wherein the secondmill is mounted to the same shaft as the window mill.
 33. The inventionas set forth in claim 32 wherein the diameter of the second mill isabout the same as the diameter of the window mill.
 34. The invention asset forth in claim 33 further comprising a third mill affixed to ashaft, the third mill being spaced from the second mill and serving toelongate the window cut by the window mill and the second mill, thethird mill also serving to dress the window formed in the pipe casing.35. The invention as set forth in claim 34 wherein the third mill is awatermelon shaped mill with about the same diameter as the window milland the second mill.
 36. The invention as set forth in claim 31 whereinthe whipstock forms a ramp surface below the angled ramp of the whipstock that is substantially parallel to the axis of the whipstock, thenon-angled whipstock ramp surface allowing the window mill and thesecond upstream mill to simultaneously cut a window, when the secondmill window cut merges with the window cut formed by the window mill,the parallel ramp surface transitions into a slightly angled ramp tofurther direct the window mill and the second upstream mill out throughthe pipe casing.
 37. A whipstock for guiding a cutting tool within acasing, comprising: a body having an axis; a guide surface on said bodyadapted for guiding engagement with the cutting tool; and said guidesurface including a first taper with a first angle to said axis and asecond taper with a second angle to said axis.
 38. The whipstock ofclaim 37 wherein said guide surface includes a third taper with a thirdangle to said axis.
 39. The whipstock of claim 38 wherein said guidesurface includes a fourth taper with a fourth angle to said axis. 40.The whipstock of claim 39 wherein said guide surface includes a fifthtaper with a fifth angle to said axis.
 41. The whipstock of claim 37wherein one of said first and second angles is substantially zerodegrees to said axis and therefore is substantially parallel with saidaxis.
 42. The whipstock of claim 37 wherein said first angle is morethan twice said second angle.
 43. The whipstock of claim 42 wherein saidfirst angle is five times that of said second angle.
 44. The whipstockof claim 40 wherein said first and fourth angles are substantiallyequal.
 45. The whipstock of claim 44 wherein said third and fifth anglesare substantially equal.
 46. The whipstock of claim 37 wherein saidguide surface includes surface hardening.
 47. The whipstock of claim 40wherein said first and fourth angles are substantially fifteen degrees,said second angle is substantially zero degrees, and said third andfifth angles are substantially three degrees.
 48. The whipstock of claim37 wherein said body has a curvature on its outside diameter whichsubstantially conforms to the curvature of the inside diameter of thecasing.
 49. An apparatus for cutting a window in a casing disposedwithin a well, comprising: a cutting assembly having a cutting assemblyaxis and including a first cutting member with a first bearing surfaceforming a first bearing angle with said cutting assembly axis; a guidemember having a guide member axis and including a guide surface with afirst tapered wedge surface having a first angle with said guide memberaxis and a second surface having a second angle with said guide memberaxis; said first bearing surface engaging said first tapered wedgesurface for deflecting said cutting member and then said first bearingsurface engaging said second surface for guiding said cutting assembly.50. The apparatus of claim 49 wherein said cutting member has a secondbearing surface forming a second bearing angle with said cuttingassembly axis.
 51. The apparatus of claim 50 wherein said second bearingangle has substantially the same angle as said second angle.
 52. Theapparatus of claim 49 wherein said cutting member includes a bearingsurface for cutting full gage.
 53. The apparatus of claim 49 whereinsaid first bearing angle is in the range of 0 to 45 degrees with respectto the axis of the cutting assembly.
 54. The apparatus of claim 50wherein said second bearing angle is in the range of 0 to 45 degreeswith respect to the axis of the cutting assembly.
 55. The apparatus ofclaim 52 wherein said bearing surface is parallel to the axis of thecutting assembly.
 56. The apparatus of claim 50 wherein said cuttingassembly further includes a second cutting member disposed apredetermined distance from said first cutting member.
 57. The apparatusof claim 56 wherein said guide surface includes a non-tapered surfaceadjacent said first tapered wedge surface, said non-tapered surfacehaving a length substantially equal to or greater than said distancebetween said first and second cutting members.
 58. The apparatus ofclaim 56 wherein said second cutting member cuts full gage.
 59. Theapparatus of claim 56 wherein said cutting assembly includes a thirdcutting member disposed a predetermined length from said second cuttingmember.
 60. A method of drilling a window in a casing disposed in a wellcomprising the steps of: releasably connecting a starter cutting memberto one end of a whipstock; engaging a bearing surface on the startercutting member with an initial wedge surface on the whipstock; disposingthe starter cutting member and whipstock within the casing;disconnecting the starter cutting member from the whipstock; deflectingthe starter cutting member into engagement with the casing; engaging asecond bearing surface on the starter cutting member with a subsequentwedge surface on the whipstock; and passing the centerline of thestarter cutting member from the inside diameter to the outside diameterof the casing.
 61. The method of claim 60 wherein the deflecting stepincludes paralleling the bearing surface of the starter cutting memberwith the initial wedge surface on the whipstock.
 62. A method ofdrilling a window in a casing disposed in a well comprising the stepsof: lowering a milling assembly releasably connected to a whipstockassembly into the casing; anchoring the whipstock assembly within thecasing; disconnecting the milling assembly from the whipstock assembly;lowering and rotating the milling assembly having at least two full gagecutting members; engaging a bearing surface on one of the cuttingmembers with an initial wedge surface on the face of the whipstock ofthe whipstock assembly; deflecting the one cutting member intoengagement with the casing; passing the one cutting member along anon-tapered surface of the whipstock until the second cutting member isadjacent the initial wedge surface; guiding the one cutting member alonga subsequent wedge surface on the face of the whipstock until the centerof the one cutting member is adjacent the inside diameter of the casing;engaging the one cutting member with a steep wedge surface; deflectingthe one cutting member against the casing until the center of the onecutting member passes to the outside diameter of the casing; and guidingthe one cutting member along another subsequent wedge surface on theface of the whipstock until the window is cut.
 63. A mill for cutting asecondary borehole in an existing borehole comprising: a body; aplurality of cutting surfaces having a angle with the axis of the body;one of said cutting surfaces including a bore for receiving a hollowshear member whereby upon shearing said shear member, said bore acts asa nozzle.
 64. The mill of claim 63 wherein another one of the cuttingsurfaces includes a bore for attachment to a coupling whereby uponmilling off said coupling, said coupling bore acts as another nozzle.65. The mill of claim 64 wherein said coupling includes a reducedportion therearound causing said coupling to mill cleanly from saidmill.
 66. A whipstock for deflecting a cutting tool into the wall of acasing, comprising: a body; and a guide surface on said body adapted forengagement with the cutting tool, said guide surface varying the degreeof deflection of the cutting tool as the cutting tool is lowered throughthe casing against said guide surface.
 67. The whipstock of claim 66wherein said guide surface includes varying tapers.